Artificial intervertebral disc having an articulating joint

ABSTRACT

An intervertebral device includes a first plate having an outer face and an inner face, and a second plate juxtaposed with the first plate, the second plate having an outer face, an inner face that opposes the first plate and a concavity that opposes the first plate. The device includes an elongated member extending from the first plate toward the second plate, the elongated member having a distal end with a spherical surface that is engageable with the concavity of the second plate for providing an articulating joint between the first and second plates. The device also includes a resilient member in contact with the elongated member for counteracting compressive loads on the plates, the resilient member being surrounded by the concavity of the second plate.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of the filing date of U.S.Provisional Patent Application No. 60/546,027 filed Feb. 19, 2004, thedisclosure of which is hereby incorporated herein by reference.

FIELD OF THE INVENTION

This invention relates generally to a spinal implant assembly forimplantation into the intervertebral space between adjacent vertebralbones to simultaneously provide stabilization and continued flexibilityand proper anatomical motion, and more specifically to such a devicethat has limited rotation using an uncaptured ball and socket joint witha partial ball having a large radius and substantially continuous radiiof curvature.

BACKGROUND OF THE INVENTION

The bones and connective tissue of an adult human spinal column consistsof more than twenty discrete bones coupled sequentially to one anotherby a tri-joint complex that consists of an anterior disc and the twoposterior facet joints, the anterior discs of adjacent bones beingcushioned by cartilage spacers referred to as intervertebral discs.These more than twenty bones are anatomically categorized as beingmembers of one of four classifications: cervical, thoracic, lumbar, orsacral. The cervical portion of the spine, which comprises the top ofthe spine, up to the base of the skull, includes the first sevenvertebrae. The intermediate twelve bones are the thoracic vertebrae, andconnect to the lower spine comprising the five lumbar vertebrae. Thebase of the spine is the sacral bones (including the coccyx). Thecomponent bones of the cervical spine are generally smaller than thoseof the thoracic spine, which are in turn smaller than those of thelumbar region. The sacral region connects laterally to the pelvis. Whilethe sacral region is an integral part of the spine, for the purposes offusion surgeries and for this disclosure, the word spine shall referonly to the cervical, thoracic, and lumbar regions.

The spinal column is highly complex in that it includes these more thantwenty bones coupled to one another, housing and protecting criticalelements of the nervous system having innumerable peripheral nerves andcirculatory bodies in close proximity. In spite of these complications,the spine is a highly flexible structure, capable of a high degree ofcurvature and twist in nearly every direction.

Genetic or developmental irregularities, trauma, chronic stress, tumors,and degenerative wear are a few of the causes that can result in spinalpathologies for which surgical intervention may be necessary. A varietyof systems have been disclosed in the art that achieve immobilizationand/or fusion of adjacent bones by implanting artificial assemblies inor on the spinal column. The region of the back that needs to beimmobilized, as well as the individual variations in anatomy, determinethe appropriate surgical protocol and implantation assembly. Withrespect to the failure of the intervertebral disc, the interbody fusioncage has generated substantial interest because it can be implantedlaparoscopically into the anterior of the spine, thus reducing operatingroom time, patient recovery time, and scarification.

Referring now to FIGS. 1-2, in which a side perspective view of anintervertebral body cage and an anterior perspective view of a postimplantation spinal column are shown, respectively, a more completedescription of these devices of the prior art is herein provided. Thesecages 1 generally comprise tubular metal body 2 having an externalsurface threading 3. They are inserted transverse to the axis of thespine 4, into preformed cylindrical holes at the junction of adjacentvertebral bodies (in FIG. 14 the pair of cages 1 are inserted betweenthe fifth lumbar vertebra (L5) and the top of the sacrum (S1)). Twocages 1 are generally inserted side by side with the external threading4 tapping into the lower surface of the vertebral bone above (L5), andthe first surface of the vertebral bone (S1) below. The cages 1 includeholes 5 through which the adjacent bones are to grow. Additionalmaterials, for example autogenous bone graft materials, may be insertedinto the hollow interior 6 of the cage 1 to incite or accelerate thegrowth of the bone into the cage. End caps (not shown) are oftenutilized to hold the bone graft material within the cage 1.

These cages of the prior art have enjoyed medical success in promotingfusion and grossly approximating proper disc height. It is, however,important to note that the fusion of the adjacent bones is an incompletesolution to the underlying pathology as it does not cure the ailment,but rather simply masks the pathology under a stabilizing bridge ofbone. This bone fusion limits the overall flexibility of the spinalcolumn and artificially constrains the normal motion of the patient.This constraint can cause collateral injury to the patient's spine asadditional stresses of motion, normally borne by the now-fused joint,are transferred onto the nearby facet joints and intervertebral discs.It would therefore, be a considerable advance in the art to provide animplant assembly which does not promote fusion, but, rather, whichmimics the biomechanical action of the natural disc cartilage, therebypermitting continued normal motion and stress distribution.

It is, therefore, an object of the invention to provide anintervertebral spacer that stabilizes the spine without promoting a bonefusion across the intervertebral space.

It is further an object of the invention to provide an implant devicethat stabilizes the spine while still permitting normal motion.

It is further an object of the invention to provide a device forimplantation into the intervertebral space that does not promote theabnormal distribution of biomechanical stresses on the patient's spine.

It is further an object of the invention to provide an artificial discthat provides free rotation of the baseplates relative to one another.

It is further an object of the invention to provide an artificial discthat supports compression loads.

It is further an object of the invention to provide an artificial discthat permits the baseplates to axially compress toward one another undera compressive load.

It is further an object of the invention to provide an artificial discthat permits the baseplates to axially compress toward one another undera compressive load and restore to their original uncompressed relativepositions when the compressive load is relieved.

It is further an object of the invention to provide an artificial discthat prevents lateral translation of the baseplates relative to oneanother.

It is further an object of the invention to provide an artificial discthat provides a centroid of motion centrally located within theintervertebral space.

It is further an object of the invention to provide artificialintervertebral disc baseplates having outwardly facing surfaces thatconform to the concave surface of adjacent vertebral bodies.

It is a further object of the present invention to provide a discreplacement device having a first element for seating against a lowerendplate surface of a superior vertebral body and a second element forseating against an first end plate surface of an inferior vertebralbody, said baseplates having disposed therebetween a partial sphericalmember having a large radius disposed in a complementary concavity suchthat said baseplates are articulatable against one another.

It is yet a further object of the present invention to provide a discreplacement device that is resistant to point loading and fatiguefailure.

It is still a further object of the present invention to provide a discreplacement device employing ball and socket type articulation using apartial spherical member wherein said partial spherical member is notcaptured.

Other objects of the invention not explicitly stated will be set forthand will be more clearly understood in conjunction with the descriptionsof the preferred embodiments disclosed hereafter.

SUMMARY OF THE INVENTION

The preceding objects are achieved by the invention, which is anartificial intervertebral disc or intervertebral spacer devicecomprising a pair of support members (e.g., spaced apart baseplates),each with an outwardly facing surface. Because the artificial disc is tobe positioned between the facing endplates of adjacent vertebral bodies,the baseplates are arranged in a substantially parallel planar alignment(or slightly offset relative to one another in accordance with properlordotic angulation) with the outwardly facing surfaces facing away fromone another. The baseplates are to mate with the vertebral bodies so asto not rotate relative thereto, but rather to permit the spinal segmentsto bend (and in some embodiments, axially compress) relative to oneanother in manners that mimic the natural motion of the spinal segment.This natural motion is permitted by the performance of a ball and sockettype joint using a partial spherical member disposed between the securedbaseplates, and the securing of the baseplates to the vertebral bone isachieved through the use of a vertebral body contact element attached tothe outwardly facing surface of each baseplate.

Preferable vertebral body contact elements include, but are not limitedto, one or more of the following: a convex mesh, a convex solid dome,and one or more spikes. The convex mesh is preferably secured at itsperimeter to the outwardly facing surface of the respective baseplate.This can be accomplished in any effective manner, however, laser weldingand plasma coating burying are two preferred methods when the mesh iscomprised of metal. While domed in its initial undeflected conformation,the mesh deflects as necessary during insertion of the artificial discbetween vertebral bodies, and, once the artificial disc is seatedbetween the vertebral bodies, the mesh deforms as necessary underanatomical loads to reshape itself to the concave surface of thevertebral endplate. Thus, the mesh is deformably reshapeable underanatomical loads such that it conformably deflects against the concavesurface to securably engage the vertebral body endplate. Statedalternatively, because the mesh is convexly shaped and is secured at itsperimeter to the baseplate, the mesh is biased away from the baseplatebut moveable toward the plate (under a load overcoming the bias; such aload is present, for example, as an anatomical load in theintervertebral space) so that it will securably engage the vertebralbody endplate when disposed in the intervertebral space. This affordsthe baseplate having the mesh substantially superior gripping andholding strength upon initial implantation, as compared with otherartificial disc products. The convex mesh further provides anosteoconductive surface through which the bone may ultimately grow. Themesh preferably is comprised of titanium, but can also be formed fromother metals and/or non-metals. Inasmuch as the mesh is domed, it doesnot restrict the angle at which the artificial disc can be implanted. Itshould be understood that while the flexible dome is described hereinpreferably as a wire mesh, other meshed or solid flexible elements canalso be used, including flexible elements comprised of non-metals and/orother metals. Further, the flexibility, deflectability and/ordeformability need not be provided by a flexible material, but canadditionally or alternatively be provided mechanically or by othermeans.

It should be understood that the convex mesh attachment devices andmethods described herein can be used not only with the artificial discsand artificial disc baseplates described or referred to herein, but alsowith other artificial discs and artificial disc baseplates, including,but not limited to, those currently known in the art. Therefore, thedescription of the mesh attachment devices and methods being used withthe artificial discs and artificial disc baseplates described orreferred to herein should not be construed as limiting the applicationand/or usefulness of the mesh attachment device.

To enhance the securing of the baseplates to the vertebral bones, eachbaseplate further comprises a porous area, which at least extends in aring around the lateral rim of each outwardly facing surface. The porousarea may be, for example, a sprayed deposition layer, or an adhesiveapplied beaded metal layer, or another suitable porous coating known inthe art. The porous ring permits the long-term ingrowth of vertebralbone into the baseplate, thus permanently securing the prosthesis withinthe intervertebral space. The porous layer may extend beneath the domedmesh as well, but is more importantly applied to the lateral rim of theoutwardly facing surface of the baseplate that seats directly againstthe vertebral body.

Some of the embodiments described herein use two baseplates each havingthe above described convex mesh on its outwardly facing surface, whileother embodiments use two baseplates each having a convex solid dome incombination with a plurality of spikes on the lateral rim of theoutwardly facing surface of the baseplates. It should be understood,however, that the various attachments devices or methods describedherein (as well as any other attachment devices or methods, such as, forexample, keels) can be used individually or in combination in anypermutation, without departing from the scope of the present invention.

The ball and socket joint, employing a partial spherical member that isnot captured, disposed between the baseplates permits rotation andangulation of the two baseplates relative to one another about acentroid of motion centrally located between the baseplates. A varietyof embodiments are contemplated. In some embodiments, the joint is usedin conjunction with a resilient member to additionally permit the twobaseplates to axially compress relative to one another. Further in eachof the embodiments, the assembly prevents lateral translation of thebaseplates during rotation and angulation.

It should be understood that the described embodiments and embodimentfamilies are merely examples that illustrate aspects and features of thepresent invention, and that other embodiments and embodiment familiesare possible without departing from the scope of the invention.

Each of the embodiments discussed herein share the same basic elements,some of which retain identical functionality and configuration acrossthe embodiments, and some of which gain or lose functionality and/orconfiguration across the embodiments to accommodate mechanical and/ormanufacturing necessities. More specifically, each of the embodimentsincludes two baseplates, each having an inwardly directed articulationsurface, having a ball and socket joint disposed therebetween employingan uncaptured partial spherical member that is established centrallybetween the baseplates. The partial spherical member has a large radiusand substantially continuous arc of curvature to minimize point loadingand reduce the risk and incidence of fatigue failure. Each of theembodiments will be understood further in light of the additionaldescriptions of the embodiments herein.

The inwardly directed articulation surface of the first baseplate isadapted such that extending thereform is a member having at its distalend a partial spherical member. The partial spherical member is definedby a convex arc that forms the articulation surface that iscomplementary to a concave articulation surface of the second baseplate.

In a preferred embodiment the longitudinally inwardly directedarticulation surface of the first baseplate comprises essentially acentrally disposed projection having a central bore for receiving and/orretaining an elongated member. The projection is sized to have adiameter less than the diameter of the inwardly directed concavearticulating surface of the second baseplate. The projection preferablyhas a cross section that is cylindrical or frustoconical.

In a preferred embodiment, the elongated member comprises essentially amushroom-shaped pin having an elongated portion and a head portion, theelongated portion thereof seated in a central bore of the firstbaseplate and the head portion, located distally, having a convex archaving a substantially constant radius of curvature A. The pin shapedmember may be fixedly engaged in the bore or may be slidably engaged inthe bore. In the embodiment in which the pin is slidably engaged in thebore, in a preferred embodiment a resilient annular member such as aresilient washer or the like is optionally deployed over the projectionof the first baseplate as a shock absorber, the resilient annular memberbeing positioned with one side facing the surface adjacent theprojection of the first member and the opposite side of the annularresilient member facing the interior of the head of the pin-shapedmember.

The elongated portion of the pin member preferably comprises acontinuous cylindrical cross section; however, the cross section mayvary toward the distal end thereof, such as by gradually or abruptlythickening near the juncture of the elongated member and the headportion, to provide structural strength.

The longitudinally inwardly directed articulation surface of the secondbaseplate is a substantially constant radii concave articulation surfaceforming a curvate socket.

The constant radii articulation surfaces are configured and sized to benestable against one another and articulatable against one another, toenable adjacent vertebral bones (against which the first and secondbaseplates are respectively disposed in the intervertebral space) toarticulate in flexion, extension, and lateral bending. Moreparticularly, the artificial disc implant of the present invention isassembled by disposing the first and second baseplates such that thevertebral body contact surfaces are directed away from one another, andthe articulation surfaces are nested against one another such that theconcave arc accommodates the convex arc.

The curvate socket defines a spherical contour that closely accommodatesthe partial spherical member for free rotation and angulation.Therefore, when seated in the curvate socket, the partial sphericalmember can rotate and angulate freely relative to the curvate socketthrough a range of angles, thus permitting the opposing baseplates torotate and angulate freely relative to one another through acorresponding range of angles equivalent to the fraction of normal humanspine rotation and angulation (to mimic normal disc rotation andangulation). Because the baseplates are made angulatable relative to oneanother by the partial spherical member being rotatably and angulatablycoupled in the curvate socket, the disc assembly provides a centroid ofmotion within the sphere defined by the partial spherical member.Accordingly, the centroid of motion of the disc assembly remainscentrally located between the vertebral bodies, similar to the centroidof motion in a healthy natural intervertebral disc.

Optionally, the end of the mushroom-shaped pin element proximal to thebaseplate, and the bore in which it is located, may be covered by avertebral body contact element disposed on or as the outside surface ofthe baseplate. In such an embodiment it is preferable to include such avertebral body contact element disposed on or as the opposing baseplatefor purposes of symmetry. Such contact elements are preferably contouredto match the contour of the surface it contacts in the intervertebralspace.

In other preferred embodiments of the present invention, anintervertebral device includes a first plate having an outer face and aninner face, and a second plate juxtaposed with the first plate, thesecond plate having an outer face, an inner face that opposes the firstplate and a concavity that opposes the first plate. The devicepreferably includes an elongated member extending from the first platetoward the second plate, the elongated member having a distal end with aspherical surface that is engageable with the concavity of the secondplate for providing an articulating joint between the first and secondplates. The device also desirably includes a resilient member in contactwith the elongated member for counteracting compressive loads on theplates, whereby the resilient member is surrounded by the concavity ofthe second plate.

In other preferred embodiments of the present invention, anintervertebral device includes a first plate having an outer face and aninner face, a second plate juxtaposed with the first plate, the secondplate having an outer face and an inner face that opposes the firstplate, and a ball and socket articulating joint provided between thefirst and second plates. The device also preferably includes a resilientmember in contact with the ball portion of the ball and socketarticulating joint for counteracting compressive loads on the plates,whereby the resilient member extends between the first and second platesand is surrounded by the socket portion of the articulating joint.

In still other preferred embodiments of the present invention, anintervertebral device includes a first plate having an outer face and aninner face, a second plate juxtaposed with the first plate, the secondplate having an outer face and an inner face that opposes the firstplate, the inner face of the second plate having a concavity, and anelongated member extending from the inner face of the first plate towardthe second plate, the elongated member being slideably attached to thefirst plate and having a distal end with a spherical surface that formsa ball and socket-like articulating joint between the first and secondplates. The device may also include a resilient member in contact withthe distal end of the elongated member for counteracting compressiveloads on the plates. The concavity of the second plate desirablysurrounds the resilient member. The elongated member may have amushroom-shaped head at the distal end thereof.

These and other preferred embodiments of the present invention will bedescribed in more detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a side perspective view of a prior art interbody fusiondevice.

FIG. 2 shows a front view of the anterior portion of the lumbo-sacralregion of a human spine, into which a pair of interbody fusion devicesof FIG. 1 have been implanted.

FIG. 3 is a cross sectional view of a first embodiment of the presentinvention, the first baseplate having an inwardly directed articulatingsurface having extending therefrom a mushroom-shaped pin element havinga partial spherical element at the distal end thereof and a secondbaseplate having a circular recess within which seats the convexstructure of the partial spherical element of the first baseplate.

FIG. 4 is a cross-sectional view of a second embodiment of the presentinvention in which the pin element is slidably engaged in a central boreof the first baseplate and further includes a resilient member disposedbetween the first and second baseplates.

FIG. 5 is a cross-sectional view of a preferred embodiment of thepresent invention.

FIG. 6 is a cross-sectional view of a preferred embodiment of thepresent invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

While the invention will be described more fully hereinafter withreference to the accompanying drawings, in which particular embodimentsand methods of implantation are shown, it is to be understood at theoutset that persons skilled in the art may modify the invention hereindescribed while achieving the functions and results of the invention.Accordingly, the descriptions that follow are to be understood asillustrative and exemplary of specific structures, aspects and featureswithin the broad scope of the invention and not as limiting of suchbroad scope. Like numbers refer to similar features of like elementsthroughout.

A preferred embodiment of the present invention will now be described.

Referring to FIG. 3, the invention is shown having a first baseplate 10and a second baseplate 30 and a pin 50. Each baseplate 10,30 has anoutwardly facing surface 12,32. Because the artificial disc of theinvention is to be positioned between the facing surfaces of adjacentvertebral bodies, the two baseplates 10,30 used in the artificial discare disposed such that the outwardly facing surfaces 12,32 face awayfrom one another. The two baseplates 10,30 are to mate with thevertebral bodies so as to not rotate relative thereto, but rather topermit the spinal segments to bend relative to one another in mannersthat mimic the natural motion of the spinal segment. This motion ispermitted by the performance of a ball and socket joint disposed betweenthe secured baseplates 10,30. The mating of the baseplates 10,30 to thevertebral bodies and the construction of the ball and socket joint aredescribed below.

More particularly, each baseplate 10,30 is a plate (preferably made of ametal or metal alloy, such as, for example, cobalt-chromium or titanium)having an overall shape that conforms to the overall shape of therespective endplate of the vertebral body with which it is to mate.Further, each baseplate 10,30 comprises a vertebral body contact element80,82 (e.g., a convex mesh, preferably oval in shape) that is attachedto the outwardly facing surface 12,32 of the baseplate 10,30 to providea vertebral body contact surface. The mesh 80,82 is secured at itsperimeter to the outwardly facing surface 12,32 of the baseplate 10,30.The mesh 80,82 is domed in its initial undeflected conformation, butdeflects as necessary during insertion of the artificial disc betweenvertebral bodies, and, once the artificial disc is seated between thevertebral bodies, deforms as necessary under anatomical loads to reshapeitself to the concave surface of the vertebral endplate. This affordsthe baseplate 10,30 having the mesh 80,82 substantially superiorgripping and holding strength upon initial implantation as compared withother artificial disc products. The mesh 80,82 further provides anosteoconductive surface through which the bone may ultimately grow. Themesh 80,82 is preferably comprised of titanium, but can also be formedfrom other metals and/or non-metals without departing from the scope ofthe invention.

Each baseplate 10,30 may further comprises at least a lateral ring (notshown) that is osteoconductive, which may be, for example, a sprayeddeposition layer, or an adhesive applied beaded metal layer, or anothersuitable porous coating. This porous ring permits the long-term ingrowthof vertebral bone into the baseplate 10,30, thus permanently securingthe prosthesis within the intervertebral space. It shall be understoodthat this porous layer may extend beneath the domed mesh 80,82 as well,but is more importantly applied to the lateral rim of the outwardlyfacing surface 12,32 of the baseplate 10,30 that seats directly againstthe vertebral body.

Each of the baseplates 10,30 comprises features that, in conjunctionwith other components described below, form the ball and socket joint.The first baseplate 10 includes an inwardly facing articulating surface18 that includes a perimeter region 20 and a projection 22 protrudingfrom the inwardly facing surface 18. The projection 22 preferably has acylindrical or frustoconical cross section. The projection 22 furtherincludes an axial bore 26 that accepts a mushroom-shaped pin 50 (orrivet, plug, dowel, or screw).

The second baseplate 30 comprises an inwardly facing articulationsurface 34 having a peripheral surface 36 and a curvate socket 38, thesocket 38 having a substantially constant radii concave articulationsurface.

Pin 50 further comprises an elongated portion 52 and a head 54, the head54 having a convex arc having a substantially constant radius ofcurvature. The arc of head 54 is such that the sphere it defines has alarge radius, thereby minimizing point loading and the risk of fatiguefailure.

The projection 22 of baseplate 10 is sized to have a diameter at least aportion of which is less than the diameter of the socket 38. Theprojection 22 preferably has a cross section that is cylindrical orfrustoconical.

In a first embodiment, the elongated portion 52 of mushroom-shaped pin50 is disposed in bore 26 of the baseplate 10 and the head 54 is nestedin socket 38. Pin 50 is fixedly engaged by force fitting, welding or thelike in bore 26. Head 54 is not captured in socket 38. Baseplates 10 and30 are at no time connected to each other in the ball and socket jointof the present invention.

Optionally, the end of pin 50 proximal to the baseplate 10, and the bore26, are covered by a vertebral body contact element 80 disposed over theoutside surface 12 of the baseplate 10. In such an embodiment it ispreferable to include a vertebral body contact element 82 on thebaseplate 30 for purposes of symmetry. Such contact elements 80 and 82are preferably contoured to match the contour of the surface it contactsin the intervertebral space.

Now referring to FIG. 4, in a preferred embodiment, pin 50 is slidablyengaged in bore 26. In this embodiment, in a preferred embodiment aresilient annular member 60 such as a resilient washer or the like isdeployed over the projection 22 (which in this embodiment is preferablycylindrical) of the first baseplate 10 as a shock absorber, theresilient annular member 60 being sized and positioned such that itfunctions as a force restoring element (e.g., a spring) that providesaxial cushioning to the device, by deflecting under a compressive loadand restoring when the load is relieved.

Now referring to FIGS. 5 and 6, in other embodiments the elongatedportion 52 of pin 50 preferably has a continuous cylindrical crosssection; however, the cross section may vary toward the distal endthereof, such as by gradually or abruptly thickening near the junctureof the elongated member 52 and the head 54, to provide structuralstrength and/or to provide a different location for resilient member 60.Now referring to FIG. 5, in a preferred embodiment resilient member 60is a continuous collar comprising a spring having a cylindrical crosssection. It is desirable, but not essential, to use a spring as theresilient member 60 because of the ability of a spring to hold itsdiameter when subjected to compressive force. In a most preferredembodiment resilient member 60 is retained in a retainer 62. Retainer 62is formed of a resilient material such as but not limited to anelastomeric material. In this embodiment elongated member 52 has afrustoconical section 56 adjacent proximal head 54 such that resilientmember 60 and retainer 62 are firmly engageable in a seat formed betweenthe frustoconical section 56 of elongated portion 52 and the end 28 ofprojection 22. As forces are applied to retainer 62, the springcomprising resilient member 60 deforms outwardly such that its diameterincreases.

In another embodiment, now referring to FIG. 6, resilient member 60 isan O-ring preferably formed of an elastomeric material. Retainer 62 is acollar such as a split collar having formed thereon an exterior groove64 to accommodate secure mounting of a resilient member 60. In thisembodiment elongated member 52 has a frustoconical section 56 adjacentproximal head 54 such that resilient member 60 and retainer 62 arefirmly engageable between the frustoconical section 56 of elongatedportion 52 and the end 28 of projection 22. As forces are applied toretainer 62, the O-ring comprising resilient member 60 deforms outwardlysuch that its diameter increases.

The substantially constant radii articulation surfaces of the head 54and socket 38 are configured and sized to be nestable against oneanother and articulatable against one another, to enable adjacentvertebral bones (against which the baseplates 10 and 30 are respectivelydisposed in the intervertebral space) to articulate in flexion,extension, and lateral bending. More particularly, the artificial discimplant of the present invention is assembled by disposing thebaseplates 10 and 30 such that the vertebral body contact surfaces 80,82are directed away from one another, and the articulation surfaces (head54 and socket 38) are nested against one another such that the concavearc of socket 38 accommodates the convex arc of head 54.

While there has been described and illustrated specific embodiments ofan artificial disc, it will be apparent to those skilled in the art thatvariations and modifications are possible without deviating from thebroad spirit and principle of the invention. The invention, therefore,shall not be limited to the specific embodiments discussed herein.

1. An intervertebral device comprising: a first plate having an outerface and an inner face; a second plate juxtaposed with said first plate,said second plate having an outer face, an inner face that opposes saidfirst plate, and a concavity that opposes said first plate; an elongatedmember extending from said first plate toward said second plate, saidelongated member having a distal end with a spherical surface that isengageable with said concavity of said second plate for providing anarticulating joint between said first and second plates; a resilientmember in contact with said elongated member for counteractingcompressive loads on said plates, wherein said resilient member issurrounded by said concavity of said second plate.
 2. The intervertebraldevice as claimed in claim 1, wherein said elongated member is slidablyattached to said first plate.
 3. The intervertebral device as claimed inclaim 2, wherein the inner face of said first plate has a projectionthat extends toward the inner face of said second plate, said projectionhaving a bore formed therein that is adapted to receive said elongatedmember.
 4. The intervertebral device as claimed in claim 3, wherein saidresilient member extends at least partially around said projection andsaid elongated member.
 5. The intervertebral device as claimed in claim1, wherein said first and second plates have respective central regionsand peripheral regions, and wherein said concavity is located in thecentral region of said second plate, and wherein said resilient memberextends between the central regions of said first and second plates. 6.The intervertebral device as claimed in claim 1, wherein said first andsecond plates comprise materials selected from the group consisting ofmetal and metal alloys.
 7. The intervertebral device as claimed in claim6, wherein the metal alloys comprise materials selected from the groupconsisting of cobalt-chromium and titanium.
 8. The intervertebral deviceas claimed in claim 1, wherein the outer face of at least one of saidfirst and second plates includes an osteoconductive surface.
 9. Theintervertebral device as claimed in claim 8, wherein saidosteoconductive surface comprises a mesh overlying the outer face of theat least one of said first and second plates.
 10. The intervertebraldevice as claimed in claim 9, wherein said mesh is deflectable undercompressive loads.
 11. The intervertebral device as claimed in claim 1,wherein said elongated member is mushroom-shaped.
 12. The intervertebraldevice as claimed in claim 1, wherein said elongated member is anelement selected from the group consisting of a pin, a rivet, a plug, adowel and a screw.
 13. The intervertebral device as claimed in claim 1,wherein the spherical surface at the distal end of said elongated memberhas a constant radius of curvature.
 14. The intervertebral device asclaimed in claim 13, wherein the concavity of said second plate has aconstant radius of curvature that is substantially similar to theconstant radius of curvature of the spherical surface at the distal endof said elongated member.
 15. The intervertebral device as claimed inclaim 1, wherein said resilient member comprises a spring.
 16. Theintervertebral device as claimed in claim 1, wherein said resilientmember comprises an O-ring made of an elastomeric material.
 17. Anintervertebral device comprising: a first plate having an outer face andan inner face; a second plate juxtaposed with said first plate, saidsecond plate having an outer face and an inner face that opposes saidfirst plate; a ball and socket articulating joint provided between saidfirst and second plates; a resilient member in contact with the ballportion of said ball and socket articulating joint for counteractingcompressive loads on said plates, wherein said resilient member extendsbetween said first and second plates and is surrounded by the socketportion of said articulating joint.
 18. The intervertebral device asclaimed in claim 17, wherein the ball portion of said articulating jointis slideably coupled with one of said plates.
 19. The intervertebraldevice as claimed in claim 17, wherein the inner face of said firstplate includes a bore and the ball portion of said articulating jointincludes an elongated member that is slideably coupled with said bore.20. The intervertebral device as claimed in claim 17, wherein the socketportion of said articulating joint comprises a concavity that is adaptedto receive the ball portion of said articulating joint, and wherein saidconcavity surrounds said resilient member.
 21. The intervertebral deviceas claimed in claim 17, wherein said articulating joint is ametal-on-metal articulating joint.
 22. An intervertebral devicecomprising: a first plate having an outer face and an inner face; asecond plate juxtaposed with said first plate, said second plate havingan outer face and an inner face that opposes said first plate, the innerface of said second plate having a concavity; an elongated memberextending from the inner face of said first plate toward said secondplate, said elongated member being slideably attached to said firstplate and having a distal end with a spherical surface that forms a balland socket-like articulating joint between said first and second plates;a resilient member in contact with the distal end of said elongatedmember for counteracting compressive loads on said plates.
 23. Theintervertebral device as claimed in claim 22, wherein said concavitysurrounds said resilient member.
 24. The intervertebral device asclaimed in claim 22, wherein said elongated member has a mushroom-shapedhead at the distal end thereof.